1. Field of the Invention
The present invention relates to blood pressure measurement system, and more particularly to an electronic blood pressure measurement system designed to mimic the characteristic response of mercury-gravity manometers.
2. Discussion of the Background Art
The measurement and monitoring of an individual's blood pressure is an important diagnostic tool in modern medicine. The knowledge of an individual's blood pressure and the periodic monitoring of their blood pressure may be useful in the prevention and treatment of a variety of conditions and/or diseases potentially stemming from high blood pressure (hypertension) or low blood pressure (hypotension). Simply stated, blood pressure is the force that blood exerts on a unit area of the blood vessel walls. During the cardiac cycle, blood pressure varies cyclically with each beat of the heart. The two blood pressure values which are of interest are the systolic and diastolic pressures. The systolic pressure is the pressure that results as a consequence of the "compressive stroke" of the heart, i.e. ventricular systole. Systolic pressure is measured after the ventricles contract to force blood from the heart. The diastolic pressure is the pressure that results as a consequence of the "intake stroke" of the heart, i.e. diastole.
Diastolic pressure is measured after the ventricles relax. Accordingly, the systolic pressure is higher than the diastolic pressure. Typically, both pressure values are represented in millimeters of mercury (mm Hg).
There are a variety of non-invasive blood pressure measurement and monitoring devices currently employed by health care professionals, and a growing number of automatic, non-invasive devices currently available for self-diagnostic purposes. The most widely accepted and utilized device, and the one which is generally utilized as a reference for other devices, is the mercury-gravity pressure manometer commonly referred to as a sphygmomanometer. The sphygmomanometer comprises a pressure cuff and a graded column of mercury. The pressure in the cuff can be varied and is balanced against the column of mercury in order to determine the individual's blood pressure. Essentially, the pressure cuff physically squeezes a part of the body where blood flow may be occluded, and a second device, such as a stethoscope, is utilized to determine when the blood flow has been occluded and when normal blood flow subsequently resumes as a result of varying the pressure in the cuff in accordance with the generally accepted procedure described below.
A technician or other health care professional utilizing the auscultatory method for determining blood pressure places the pressure cuff around an available limb, typically the arm of an adult, and raises the pressure in the cuff until the flow of blood in the limb is occluded and no pulse is felt. The health care professional palpates the artery and increases the pressure in the cuff until the point of obliteration of the pulse is felt by the health care professional. Thereafter, the pressure in the cuff is reduced to zero and then reinflated to approximately 30 mm Hg above the point of obliteration. The pressure in the cuff is then gradually released until the point where a Korotkoff sound is heard by the health care professional with the assistance of a stethoscope. At this point, the pressure in the cuff is equal to the individual's systolic blood pressure. The health care professional makes a mental note of this pressure by reading the graded mercury column. To read this number accurately, the pressure in the cuff must dwell long enough at the correct systolic pressure for the heart to reach a compressive stroke so that a pulse is generated. If the pressure in the cuff is released too rapidly, the systolic pressure reading may be low by as much as the drop in the cuff pressure that occurred between consecutive heart beats. Conversely, if the pressure in the cuff is released too slowly, venous congestion, i.e. arteries constrict faster than veins dilate, may cause an error in the systolic pressure reading. The optimum rate of pressure release in the cuff is 2 to 3 mm Hg per heartbeat. This pressure release rate limits the average systolic pressure error to a range from about 0 to about 1.5 mm Hg. Once the systolic pressure is noted, the health care professional continues to release the pressure in the cuff. As the pressure in the cuff is reduced, the Korotkoff sound changes through several phases. The first phase begins at the systolic pressure and is recognized by a "sharp thud" sound. This sound persists as the pressure in the cuff continuously decreases until the second phase is reached (typically about a 10 mm Hg drop). The second phase is recognized by a "blowing" or "swishing" sound. The second phase Korotkoff sound persists for about a 10 mm Hg drop in cuff pressure. The third phase is characterized by a pulse having a softer thud than the thud heard in the first phase. As the pressure in the cuff continues to decrease, the fourth phase occurs and is characterized by a softer blowing sound that becomes progressively quieter as the pressure in the cuff is reduced until the Korotkoff sound disappears, thereby marking the beginning of the fifth phase. The beginning of the fourth phase marks the first possible diastolic pressure and the beginning of the fifth phase marks the second possible diastolic pressure. Depending upon the health care professionals school of practice, the health care professional simply notes either of these two values by reading the graded mercury column and making a mental note of the value.
Mercury-gravity pressure manometers are extremely accurate devices and offer a number of advantages over other existing blood pressure measuring devices or systems. For example, mercury-gravity pressure manometers provide a clear indication of the cuff pressure release rate. The release rate may be easily determined by watching the mercury slowly descend in the graded column. Another advantage of the mercury-gravity pressure manometer is its response characteristics. Because of the mass of mercury, the hydraulic flow design, and the restrictive air vent at the top of the column, the mercury-gravity manometer responds precisely and smoothly to pressure changes. Accordingly, the mercury in the column will not react, i.e. jump or oscillate, in response to rapid fluctuations in the blood pressure which may be caused by internal or external influences. This characteristic is critical for ensuring proper blood pressure measuring technique thereby providing for extremely accurate and repeatable measurements. The mercury-gravity pressure manometer also has an advantage in that no re-calibration is necessary. While mercury-gravity pressure manometers offer these advantages, their use may be somewhat restricted by size and necessary vertical orientation of the mercury column. For example, if a patient is in a confined area, it may be difficult to utilize a portable mercury-gravity pressure manometer because for normal operation the mercury column has to be positioned vertically and the confined area may only allow the mercury column to be positioned horizontally. In addition, mercury-gravity pressure manometers may not be suitable in gravity-free or reduced gravity environments. For example, mercury-gravity manometers may not be suitable in space applications wherein the gravitational field is not equivalent to the earth's gravitational field.
Other devices may be utilized to measure and monitor blood pressure. Aneroid manometers, for example, may be utilized. In an aneroid manometer, the column of mercury is replaced by a deflectable element such as a Bourdon tube or diaphragm capsule. Unlike mercury-gravity pressure manometers, however, aneroid manometers require frequent calibration because of the wear on the deflectable element and its movement. Additionally, aneroid manometers generally cannot match the desired response characteristics provided by mercury.
Automated blood pressure monitoring has rapidly become an accepted, and in many cases, an essential aspect of human medical treatment. Automated blood pressure monitoring devices are now a conventional part of the patient environment in emergency rooms, intensive and critical care units, and in the operating theater. Less expensive automated blood pressure monitoring devices are also gaining wide acceptance in the home health care market.
An exemplary automated blood pressure monitoring device typically utilizes the point of peak arterial counter pressure oscillations as an indicator of mean arterial pressure. Essentially, the device identifies the peak arterial counter pressure oscillations to determine the mean arterial pressure and then calculates the systolic and diastolic blood pressure values utilizing known algorithms. The monitoring device comprises an automatic inflation pump which inflates a standard pressure cuff to a predetermined pressure limit. If a pulse is detected via a sensor, the pressure cuff is pressurized to a higher value. Once no pulse is detected by the sensor, the pressure in the cuff is decreased incrementally. There is a correlation between the mean arterial pressure and the lowest cuff pressure that yields maximum arterial counter pressure oscillations; accordingly, the mean arterial pressure may be determined by measuring the counter pressure oscillations as the cuff pressure is reduced in discrete increments. A pressure transducer in the pressure cuff may be utilized to detect the arterial counter pressure oscillations. A processor then calculates the systolic and diastolic blood pressures from the mean arterial pressure using the known algorithms.
A typical problem associated with automated blood pressure monitoring devices is that they are not foolproof. The individual undergoing blood pressure measurement and/or continuous monitoring must have a strong enough pulse to be detected by the pressure transducers in the presence of background noise. Background noise can come from the pressure transducers, the electronic support circuitry, vibrations in the room, muscle flexing by the patient, and other sources. Accordingly, the individual should remain motionless during this process because slight movement will cause pressure changes that will be interpreted by the pressure transducers as a pulse. Essentially, the pressure transducers are too responsive in reacting to pressure changes thereby making them oversensitive to transient variations. Therefore, the very individuals that require accurate and continuous blood pressure monitoring are the ones that will get the least accurate readings from automated blood pressure monitoring devices because these individuals often have irregular heart rates and their pulses are not crisp and strong enough for the automated devices to correctly detect and recognize.
A problem associated with automated blood pressure monitoring devices that measure arterial counter pressure oscillations such as described above is the introduction of error causing artifacts due to inadequately or inaccurately sensed counter pressure oscillations. Generally, the arterial counter pressure oscillation signals from the sensor constitute relatively low level, noisy signals superimposed on the much larger, sometimes poorly regulated cuff pressure baseline. Accordingly, these signals are typically passed through band pass filters which are designed to pass the oscillation signals but block noise on the high frequency end and the base cuff pressure and slower variations thereof at the lower end. However, this band pass filtering may contribute further inaccuracy to the counter pressure oscillation detection and measurement process.
Other problems associated with automated blood pressure monitoring devices include a lack of speed and versatility, blood pressure measurement not based on Korotkoff sounds, and prohibitive cost. Automated blood pressure monitoring devices generally cannot be rapidly reset for reuse, nor do they allow the health care professional latitude in use. For example, situations may arise wherein the health care professional wants a quick blood pressure reading based on a transitory condition and the intermittent, fixed interval measurement performed by automated devices simply cannot react quickly enough to catch what may constitute a key blood pressure reading. Automated blood pressure monitoring devices do not utilize Korotkoff sounds for determining blood pressure. Accordingly, the blood pressure readings are typically not as accurate as those that are achieved with standard mercury-gravity pressure manometers. Automated blood pressure monitoring devices are typically expensive devices which unnecessarily raise the cost of health care.
The electronic blood pressure measurement system of the present invention overcomes the limitations associated with the currently utilized measurement devices described above.